Language and number in Williams Syndrome and Down's Syndrome : from infant precursors to the mature phenotype
This thesis is an examination of language and number in two atypically developing groups, Williams Syndrome (WS) and Down's Syndrome (DS). These groups were chosen because their cognitive profiles in adulthood differ significantly. It is already known that language is a relative strength in WS but that it is poorer than non-verbal ability in DS. The precursors to both language and number ability were studied in 24-36 month old infants and performance at this stage was compared with that in the steady state, by testing older children and adults, aged 9-35 years. Similar age-appropriate tests were used with both groups so that performance in the steady state could be compared with that in infancy. Specific subdomains of language and number were assessed to investigate whether the pattern seen in the adult steady state was also present in infancy, or whether the mature phenotype is a product of the different developmental trajectories followed by each group. The overall cognitive profile of infants with WS and DS did not differ significantly, despite clear distinctions between the adult profiles. However, their performance on number and language tasks did differ in infancy. While in adulthood WS performance on number tasks was poorer than that of DS, in infancy this pattern was reversed. For language, infants with DS exhibited a large discrepancy between productive and receptive vocabulary. A more even pattern was present for the WS group. In adulthood, vocabulary was better in WS than DS but both groups had problems with syntactic structures. Taken together these results suggest that it is not possible to derive the pattern of infant performance from the steady state in adulthood. The developmental trajectories from precursors to mature phenotype need to be thoroughly charted in atypical populations because the study of development, not just the characterisation of the endstates, is crucial.